The invention relates to a power supply which includes an inverter and a voltage cascade (voltage multiplier) for converting an input voltage applied to the inverter into an output DC voltage (supply voltage), notably a high voltage for X-ray generators or rotating X-ray systems, for example, in computed tomography apparatus, or for single tank generators. The invention also relates to an X-ray system of this kind.
Generally speaking, X-ray systems include an X-ray source with an X-ray tube for generating X-rays, as well as an X-ray generator with a power supply (high-voltage generator) which includes a power supply unit and delivers the high voltage necessary for operation of the X-ray tube. When the X-ray source and the parts generating the high voltage are combined so as to form one structural unit, a so-called single tank generator is obtained.
The operation of X-ray tubes requires anode voltages of up to, for example approximately 150 kV in the case of single-pole power supply or approximately +/xe2x88x9275 kV in the case of double-pole power supply between the anode and the cathode. Numerous requirements are imposed as regards these voltages or the circuit generating these voltages. In order to avoid fluctuations of the X-ray intensity produced, the voltages should be as constant as possible and have a small ripple only. As the output powers become higher, an ever higher output capacitance is then required so as to smooth voltage at a given switching frequency of the inverter.
For fast control of the output voltage and of the output current (for example, in the pulsed mode of operation of the X-ray tube) and notably for fast reduction of the output voltage in the case of small currents, however, an as small as possible output capacitance is to be pursued; this capacitance should also be as small as possible in order to limit the loading of the X-ray tube in the case of a fault.
Furthermore, for application-specific or construction-specific reasons (not described herein) it may also be advantageous to use a single-pole supply voltage. The voltage of, for example 150 kV to be realized in that case imposes special requirements as regards the design of the high-voltage tank.
A low weight and a small volume are especially important in the case of use in single tank generators and in rotating X-ray systems such as, for example, computed tomography apparatus (CT apparatus). These are notably systems that operate with a speed of rotation of several revolutions per second, for example, sub-second scanners in which centrifugal forces of 30 g or more are liable to occur.
U.S. Pat. No. 4,995,069 discloses a power supply for an X-ray tube, in particular for a CT apparatus, in which an AC voltage applied to the input is first rectified by means of a rectifier. In order to generate a double-pole supply voltage for the X-ray tube, for each pole a respective inverter is connected to the rectifier in order to generate an AC voltage and a transformer for stepping up the AC voltage to an intermediate voltage is connected to the output thereof. For each pole the necessary anode voltage or cathode voltage is generated from said intermediate voltages by means of a respective voltage cascade. In order to save weight on the rotating part of the CT apparatus, the input voltage source, the rectifier, the inverters and the transformers are arranged on a stationary part, the secondary terminals of the transformers being connected to the inputs of the voltage cascades via slip rings and slip contacts. This approach, however, is considered to be disadvantageous because the slip rings and slip contacts are subject to comparatively severe wear, notably in the case of high rotary speeds and/or high electric powers, and require frequent maintenance.
Therefore, it is an object of the invention to provide a power supply which is suitable particularly for operation of an X-ray tube in a rotating X-ray system and has a particularly low weight and a small volume in relation to its output power.
Furthermore, it is an object of the invention to provide a power supply of the kind set forth in which the output voltage and the output current can be rapidly controlled also in the case of a high output power and with a small ripple.
Finally, the invention aims to provide a power supply which is also advantageously suitable for generating a single pole supply voltage.
In order to achieve these objects, there is provided a power supply of the kind set forth which is characterized in that it includes a control circuit for generating a switching voltage whereby the inverter can be driven at a switching frequency such that a resonance current is imposed in the voltage cascade.
An advantage of this solution resides in the fact that in the resonant mode a variation of the output power with a very steep characteristic is possible by way of a comparatively small variation of the switching frequency and/or the pulse duty factor with which the inverter is driven.
The saving of weight also enables attractive implementation of mobile X-ray systems.
On the other hand, such a saving of weight can be traded off against proportions enabling a particularly small ripple or a high output power to be obtained.
A further advantage of this solution occurs notably in the case of high output powers, because the inherent loss power is then better distributed among the components so that they are subject to significantly less severe loading.
The present invention offers the advantage that in comparison with known power supplies with voltage multipliers in a cascade configuration (which do not operate in the resonant mode) the transformers can be constructed so as to be significantly smaller and nevertheless have the same output capacitance when a further reduction of the ripple of the output voltage is dispensed with. This offers a significant reduction of the weight of the power supply or a significantly more attractive ratio of the output power of the circuit on the one hand to its weight on the other hand, that is, in comparison with the known voltage cascades. Notably in the case of rotating X-ray systems, for example, computed tomography apparatus, it is thus possible to mount the X-ray source and all components of the power supply advantageously on the rotating part, so that only a comparatively low input voltage must be conducted via the slip rings and slip contacts.
The ratio of the output power to the weight of the circuit can be further improved by other embodiments of the invention. Such improvement is based essentially on the fact that, for example, the transition from one to 3 phases reduces the ripple of the output voltage by a factor of 6 for the same output capacitance of the voltage cascade. Conversely, for the same ripple the output capacitance could be reduced accordingly if (as described above), for example, particularly fast control of the output voltage or the output current for the X-ray tube is important in the pulsed mode of operation.
In another embodiment the invention enables simple control of the resonance current in the cascade, and hence of the output power, by variation of the driving of the inverter. This embodiment is particularly suitable for the pulsed mode operation of an X-ray tube.